This disclosure relates to a discharge lamp, such as an automotive headlamp, and more particularly to an ignition aid therefor. In addition, the disclosure relates to an ignition aid that serves as a support for controlling the gap between an arc tube and surrounding shroud so that a maximum thermal stress of the arc tube is within a desired range. It will be appreciated, however, that the disclosure finds application in related environments and applications.
High intensity discharge (HID) lamps require very high ignition voltages to break down a gap and initiate an arc between spaced electrodes. For example, an ignition pulse on the order of approximately 25 kV is required to start an automotive discharge headlamp. As will be appreciated, this high ignition voltage presents a challenge to lamp designers to provide a ballast that meets these requirements, and typically results in increased lamp component costs. In addition, electromagnetic interference (EMI) becomes an issue at these high voltages and often requires a ground or shield to contain the EMI and not adversely impact nearby electrical components.
In addition, ceramic discharge headlamps have thermal stress issues. One manner of addressing the thermal stress is to use a shroud that fits closely around the arc tube. The spacing between the shroud and the arc tube must be small, for example on the order of approximately one (1) millimeter. However, it is difficult to maintain this spacing during the assembly process.
One known ignition aid for reducing a starting or ignition voltage for a discharge lamp is shown and described in commonly owned U.S. Pat. No. 4,053,809. That disclosure is directed to a short arc discharge lamp where the spaced electrodes are sealed in a thick-walled quartz envelope. The electrode gap or arc length is about 2 or 3 millimeters in an exemplary 300 watt metal halide gas short arc discharge lamp. The tungsten wire electrodes are received in an ionizable fill which includes an inert gas such as argon and a halogen or metal halide such as indium iodide. An elongated electrically conductive member is connected to one of the electrodes or inleads, and the conductive member extends toward the other electrode to serve as a starting device, or ignition aid. One embodiment of the starting device described therein includes a wire welded at one end to a first electrode and connected at an opposite end to a metal strap that encircles a stem of a second electrode. The starting device is spaced closer to the second electrode than the arc gap between the electrodes. In this manner, the starting voltage for the lamp is significantly reduced.
Another approach for a starting aid is taught in U.S. Pat. No. 5,541,480 where a high pressure discharge lamp employs a metal coating on an outer surface of a ceramic wall of the lamp. More particularly, the metal coating is a high temperature metal sintered on the ceramic wall that serves as the starting aid. Unfortunately, this arrangement requires a high temperature material in order to survive the sintering process and the process is relatively expensive in order to maintain good contact.
Yet another starting aid arrangement is disclosed in US2005/0042967 A1 which uses a conductive antenna coil wrapped around an arc tube in order to reduce the breakdown voltage of the lamp fill gas. The coil also provides containment protection to prevent damage to an outer bulb if the arc tube were to rupture.
A need exists for substantially reducing the ignition voltage necessary to start a discharge lamp, and also to support a shroud in closely spaced relation with the arc tube such that the maximum thermal stress of the arc tube is maintained within a desired range.